This invention relates generally to molded case circuit breakers and, more particularly, to a magnetic structure for calibrating a circuit breaker for tripping at a precise value of current.
A circuit breaker is a very useful device for interrupting a circuit under various conditions. For example, there are circuit breakers which interrupt the circuit when current flow of a certain magnitude flows through the breaker for a given period of time. Units of this type employ a thermal unit which responds to current flow over a period of time so that when current of a certain magnitude flows through the circuit breaker for a given period of time, the thermal unit responds by tripping the breaker thereby interrupting the circuit. Calibration is typically done by means of an adjusting screw.
In other types of circuit breakers, only a magnetic structure is used for tripping the breaker in response to current of a specified magnitude. In these circuit breakers, a magnetic unit is employed to trip the breaker in response to an over current condition, that is, current which exceeds a preselected current magnitude which is related to the rating of the breaker.
These breakers have adjustable settings so that a range of tripping current levels is available. Each of these settings represent a predetermined level of current. These levels are determined by design and are verified and set by calibrating the breaker. The breakers are calibrated by adjusting the distance between the armature and the magnet, by adjusting the biasing force on the armature, or both. This is normally done at the low setting and the high setting. The intermediate settings should be at their predetermined levels once the low and high setting is set.
At high settings, which normally represent large gaps between the armature and the magnet, calibration is more responsive to varying the biasing force. At low settings, which normally represent small gaps between the armature and the magnet, calibration is more responsive to the size of the gap.
Adjusting the biasing force is normally accomplished by adjusting a spring force on the armature. Adjusting the gap between the armature and the magnet is normally accomplished by moving the armature toward or away from the magnet. Since both of these adjustments are being made on the armature, each has a counteracting effect on the other making adjustment difficult and time consuming.
Accordingly, it is an object of the present invention to provide a simple structure for calibrating the tripping current of a circuit breaker.
Another object of the present invention is to provide a simple magnetic structure for a circuit breaker which is easily calibrated.
Yet another object of the present invention is to provide a simple magnetic structure for calibrating a circuit breaker which is useful for both high and low adjustments of the circuit breaker.